The present invention relates to a method of and a system for detecting a defect particularly suitable where the surface of that part of the object or body which is under examination is substantially a surface of revolution. An example of such a part under examination is a mouth portion of a beverage bottle such as a bottle for beer, other alcoholic liquors, soft drinks or foods.
If a bottle for alcoholic liquors, soft drinks or foods has a defect such as scratches, cracks or breakages at its mouth, then it is impossible or difficult to completely seal the mouth and problems in food hygiene occur. Thus, it is necessary to detect a defect and to reject the bottles having a defect. It is also necessary to detect a defect at other parts, such as the bottom of a bottle for maintaining food hygiene. There are similar needs in connection with cans.
A variety of methods of detecting defects in bottles have been proposed in the art; however, they are sometimes not satisfactory because of their complexity in mechanical construction, low inspection accuracy or reliability, low inspection rate and the like.
The applicant proposed in Japanese Patent Application No. 118607/1981 (Laid-open No. 21146/1983) an inspection method in which the image of a bottle bottom or the like is divided into sectors by radially extending lines, and the sums of the values indicative of the brightness of the pixels in the sectors are compared with each other and presence or absence of a defect is determined in accordance with the result of comparison. This method eliminates the need for rotating either the bottle or the image sensor, and brings about improvement in the inspection accuracy. But there are certain circumstances where the configuration or position of a defect is such that the effect of the defect is distributed between two adjacent sectors, in which case detection of the defect is difficult and may fail.